Hi eTess. If you are really worried about security then I suggest not using the facial recognition. As a note, Kobo did not develop the facial recognition software, but it comes bundled with Android.
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The best form of security is the PIN or Pattern Lock.

As an added bonus, try turning on encryption. This will prevent any random person from reading the contents of your device if you are to ever loose it!

Thanks so much for the feedback! I had pretty much ruled it out for the very reasons you describe...I would be curious to see if my sister could open my Arc if I used my pic...I have gone with one of the two safer options you mentioned.

I'm also going to look into the encryption - just worried it will lock me out accidentally!